LECTURES MATH370-08C

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1 LECTURES MATH370-08C A.A.KIRILLOV 1. Groups 1.1. Abstract groups versus transformation groups. An abstract group is a collection G of elements with a multiplication rule for them, i.e. a map: G G G : (g 1, g ) g 1 g. The sign of group multiplication in concrete cases can denote either addition +, or multiplication (or ), or composition, etc. The group multiplication satisfies three axioms: a) there exists a unit element e G such that e g = g e = g for any g G; b) for any g G there exists an inverse element 1 g such that 1 g g = g 1 g = e; c) for any g 1, g, g 3 G the associativity relation holds: (g 1 g ) g 3 = g 1 (g g 3 ). A transformation group is a collection G of invertible transformations of some set X which: a) contains the identical transformation Id; b) together with any transformation T contains the inverse transformation 1 T ; c) together with any two transformation T 1, T contains their composition T 1 T defined as T 1 T (x) = T 1 (T (x)). Any transformation group G defines an abstract group G abs. Namely, as a set, G abs coincides with G but we forget about the set X and about the meaning of T G as transformations of X. The only thing which remains is a law of multiplication of elements of G abs. The existence of unit (= Id) and the inverse is postulated by definition. As for associativity relation, it is a god-given property of composition: T 1 (T T 3 ) = (T 1 T ) T 3. Being applied to an x X both parts give T 1 (T (T 3 (x))). A homomorphism of one abstract group G to another abstract group H is a G map ϕ : G H such that ϕ(g 1 g ) = ϕ(g 1 ) H ϕ(g ). In this case we have also ϕ( 1 g ) = ϕ(g) 1 and ϕ( G e) = H e. If a homomorphism ϕ : G H is an invertible map, it is called isomorphism and we say that G and H are isomorphic. Many different transformation groups may correspond to isomorphic abstract groups. Date: Sept 008. Key words and phrases. Group, subgroup, G-set, stabilizer, normal subgroup,. 1

2 A.A.KIRILLOV 1.. Subgroups, G-sets, cosets and homogeneous spaces. A set H G is called a subgroup in a group G, if the following conditions are satisfied: 1. H contains the unit e;. if h H then 1 h H; 3. if h 1, h H, then h 1 G h H. The number of elements in G is called the order of G and is denoted G. Lagrange Theorem: H is a divisor of G (the proof is given below). Corollary: if G is a prime number, G has no proper subgroups. We say that X is a left G-set if the map G X X : (g, x) g x is defined and satisfies the axioms: e x = x for all x X, (g 1 g ) x = g 1 (g x) for all x X and g 1, g G. Denote by L(g) the transformation x g x. Then the axioms above mean that the correspondence g L(g) is a homomorphism of G into the group of transformations of the set X. Indeed, they mean L(g 1 g ) = L(g 1 ) L(g ) and L(e) = Id. Instead of saying X is a left G-set one often use the expression: G acts on X from the left. The definition of a right G-set Y (or the right action of G on Y ) is analogous: there is a map Y G Y : (y, g) y g satisfying axioms y e = y for all y Y, y (g 1 g ) = (y g 1 ) g for all y Y and g 1, g G. Define by R(g) the transformation y y g. Then the right action is a so-called antihomomorphism of G into the group of transformations of the set X. It has the properties: R(e) = Id, R(g 1 g ) = R(g ) R(g 1 ). Example. Let X be the set of column vectors, Y be the set of row vectors and G be the set of invertible square matrices of size n over some field K. Then the ordinary matrix multiplication makes G a group, X is a left G-set, and Y is a right G-set. Remarks. a) By default the G-action means a left G-action. b) For abelian (commutative) groups there is no difference between left and right action. c) From any left G-set we can easily manufacture a right G-set and vice versa. Namely, if g L(g) is a left action, then g L( 1 g ) is a right action; and if g R(g) is a right action, then g R( 1 g ) is a left action. Let X be a left G-set. Choose any point x 0 X and consider the map p : G X : g g x 0. The set H = 1 p (x 0 ) is a subgroup in G (check it!). It is called the stabilizer of x 0 in G and is denoted by Stab G (x 0 ). The G-set X is called homogeneous, if for any two points x 1, x X there exists an element g G such that g x 1 = x. In this case we also say that G acts transitively on X.

3 LECTURES MATH370-08C 3 Let X be an homogeneous G-set and H = Stab G (x 0 ). The sets C x = 1 p (x) for x X are called left H-cosets in G. It is clear (is it?) that they are disjoint and cover the whole group G. The set of all left H-cosets in G is denoted by G/H. (For the set of all right cosets - define them yourself - the notation H\G is used). Lemma 1.1. a) Let g x be any representative of a left H-coset C x G; then C x = g x H. b) All left H-cosets have the same cardinality H. c) We have G = H X Normal subgroups and quotient groups. We saw that for any pair of groups G H the number G is always divisible by H and the ratio is G/H. Does it mean that G/H is a group by itself? The answer is: no in general, but for special cases - yes. The product C 1 C of two left cosets is always defined but usually it is not a coset, but a union of several cosets. There are two lucky exceptions: the case of abelian group G and the case of normal subgroup H. A subgroup H is called normal in G if for all g G we have g H 1 g = H. (Note, that in an abelian group any subgroup is normal). In these cases every left H-coset is also a right H-coset because g H = H g. Therefore, we have: C 1 C = g 1 H g H = g 1 g H H = g 1 g H. Thus, the product of two cosets is again a coset and we have a multiplication law in G/H. It satisfies the group axioms and so G/H is a group by itself. It is called factorgroup or quotient group. Assume G has a normal subgroup G 1 and let G denote the quotient group G/G 1. Can we reconstruct G, knowing G 1 and G? This interesting question is still not completely solved. But if it were so, the study of all groups would be reduced to study simple groups which have no proper normal subgroups Direct and semidirect products. There are several ways to construct new groups from given ones. A direct product G 1 G of two groups G 1 and G is defined as follows: as a set, it is a direct product of sets, i.e., collection of pairs (g 1, g ), g i G i. The group law is defined componentwise: (g 1, g ) (g 1, g ) := (g 1 1 g 1, g g ) where 1 and are group laws in G 1 and in G respectively. Note, that both G 1 and G can be considered as normal subgroups of G 1 G : G 1 G 1 {e}, G {e} G 1. Assume that a group G has a normal subgroup G 1 G and for any coset C G/G 1 we can choose a representative g C C so that these representative form a subgroup G G. Of course, this subgroup must be isomorphic to the quotient group G/G 1.

4 4 A.A.KIRILLOV Lemma 1.. The group multiplication establishes the set isomorphism: (g 1, g ) g 1 g of G 1 G to G. In these coordinates the group law in G takes the form: ( (x 1, x ) (y 1, y ) := x 1 (x y 1 x 1 ), x y ). We see that the abstract group G can be reconstructed if we know the following data: 1. Abstract groups G 1 and G.. For any g G an automorphism A(g ) of the group G 1, such that A(e) = Id and A(g g ) = A(g ) A(g ). Namely, we define the new group, the so-called semidirect product G 1 G (or G G 1 ) as follows. As a set, G 1 G is a direct product G 1 G. The group law is defined by the rule: (g 1, g ) (g 1, g ) 1 := (g 1 (A(g )g 1, ) g g. Remarks. 1. In this construction G 1 and G play non-symmetric role what is reflected in notation.. It can happen that both G 1 and G are abelian, but G is not. A good example: the group G of isometries of R n is a semidirect product of a normal subgroup G 1 of translations and a subgroup G of isometries, preserving the origin (rotations around origin and reflections in mirrors, containing origin). 3. In the case we have started the automorphism A(g ) has the form g 1 g g 1 1 g. It allows us to remember the group law in G 1 G, considering the group G as generated by G 1 and G with relations g g 1 = A(g )g 1 g. (See next section) Examples of transformation groups. a) Let X n be a finite set of n elements. E.g., X n = {1,, 3,..., n}. The set of all invertible transformations of X n forms a group S n of all permutations of n objects a.k.a symmetric group. b) Consider the set Y n consisting of n points labelled by ±1, ±,..., ±n. Let C n be a collection of all permutation s S n such that s( k) = s(k). It is a subgroup in S n. It is isomorphic to S n S n c) Let M be a metric space with a distance d(x, y). The set of all invertible maps T which preserve the distance: ( ) d T (m 1 ), T (m ) = d(m 1, m ) for all m 1, m M, form a group Iso(M) of isometries of M. Some particular cases are: c 1 ) Let n denote a regular simplex in R n, then the group Iso( n ) is isomorphic to S n+1 ; c ) Let n denote a cube in R n, then, as abstract group, Iso( n ) is isomorphic to C n. c 3 ) Let S n 1 be a unit sphere in euclidean space R n. Then the group Iso(S n 1 ) is isomorphic to the group O(n, R) of all real orthogonal matrices of size n.

5 LECTURES MATH370-08C 5 c 4 ) Let R n be the euclidean space of dimension n. Then Iso(R n ) is isomorphic to the semidirect product O(n, R) R n Presentation of groups. A way to define an abstract group is so-called presentation: P = {g 1, g,... R 1, R,... }. where R i are equations of the form g k 1 i 1 g k i g k N in = e. Call a group G a P-group if it generated by elements g 1, g,... satisfying all relations R 1, R,.... The following remarkable theorem is true. Theorem 1.3 (Universal P -group). For any presentation P there exists a universal P -group denoted (G P ; ḡ 1, ḡ,... ) such that for any P -group (G; g 1, g,... ) there is a unique homomorphism ϕ : G P G such that ϕ(ḡ i ) = g i. Examples. 1. For P = {g 1 we have G P Z;. For P = {g 1 g 1 = e, g1 5 = e we have G P {e}; 3. For P = {s 1, s s 1 = s = (s 1 s ) 3 = e} we have G P S 3 ; 4. For P = {g 1, g g 1 g g1 1 g 1 = e} we have G P Z Z; 1.7. Conjugacy classes. Every non-abelian group has many non-trivial automorphisms. Namely, every non-central element x G defines an automorphism Indeed, A(x) : g x g x 1. A(x)(g 1 g ) = x (g 1 g ) x 1 = x g 1 x 1 x g x 1 = A(x)(g 1 ) A(x)(g ), A(x) ( g 1) = ( A(x)(g) ) 1 and ( A(x)(e) = e. These automorphisms are called inner. They acts on group elements, subsets, subgroups etc. Two objects are called conjugate if one is obtained from another by an inner automorphism. Collection of all objects conjugate to a given one is called a conjugacy class. The conjugate objects have many common features. For example, group elements from the same conjugacy class have the same order. Two conjugate subgroups are stabilizers of two different points of the same homogeneous space.. Rings and fields.1. definition of rings and fields. Consider a set R with two operations: addition + and multiplication. Often instead of a b the simple notation ab is used. The two operations are subordinate to conditions: 1. (R, +) is an abelian group with the neutral element 0.. Multiplication is distributive: a (b+c) = a b+a c and (b+c) a = b a+c a. Remark. Multiplication may subject to additional conditions: commutativity, associativity, existence of neutral element 1 and an inverse element a 1 for all a 0. If one of these properties is present, it must be mention in the definition and reflected in the name: commutative ring, associative ring, division ring. If all the properties hold, R is called a field; the associative division ring is called a skew-field.

6 6 A.A.KIRILLOV Many fields consists of ordinary real or complex numbers. Then they necessarily contain the field Q of rational numbers. Examples of rings: Z, nz, Z/(nZ), Z[ 1 ], Q, R, C, H. From these examples Q, R, C and Z/(pZ) for p prime are fields; H is a skew-field. The notion of a subring of a ring R is defined naturally: it is a subset of R, closed under both ring operations. A subring I R is called a left ideal, if I R I; a right ideal, if R I I; a two-sided ideal, if I R I & R I I. E.g. Z/(nZ) is a two-sided ideal in Z. For a ring R and a two-sided ideal I R the quotient ring or factor ring R/I is defined. As an abelian group, it is a quotient group R/I and the multiplication is defined in a standard way: [a + I] [b + I] = [ab + I]. Example: Z/nZ is a factor ring of Z... Matrix ring. For any ring R and any natural number n we construct the ring Mat n (R), consisting of square matrices of size n n with entries from R. Shortly this matrix is denoted by A = a ij where i and j run from 1 to n. The full notation is A = a 11 a 1 a 1n a 1 a a n a n1 a n a nn The ring operation are given by the rules: n A + B = a ij + b ij, AB = a ik b kj. (.1) If R is associative, so is Mat n (R). But it is not commutative for n > 1 even if R is. Most associative rings are isomorphic to subrings of Mat n (R) for an appropriate R and n.. k=1 3. Vector spaces, modules and algebras Some algebraic structures we want to study, need for their definition an auxiliary algebraic structure, a ring or a field Vector spaces. A vector space over a field K is an abelian group (V, +) endowed by an action of a field K. This means that a map K V V : (λ, v) λv is given, which satisfied the conditions: λ(v 1 + v ) = λv 1 + λv ; (λ + µ)v = λv + µv; (λµ)v = λ(µv). Usually elements of K are called numbers and elements of V are called vectors. In most cases K = R or K = C; then V is called real or complex vector space. We denote numbers by lower case Greek letters α, β,...λ, µ, ν... and vectors by lower case Roman letters, sometimes with an arrow above: u, v, w... The

7 LECTURES MATH370-08C 7 vector spaces and related structures: linear operators, polylinear forms, tensors etc constitutes the overwhelming majority of all application of algebra. Definition 3.1. A linear combination of vectors v 1,..., v n is an expression of the form λ 1 v 1 + λ v + + λ n v n. It is called trivial if λ 1 = λ = = λ n = 0. Otherwise it is called non-trivial. Definition 3.. Vectors v 1,..., v n are called linearly dependent (or simply dependent) if there is a non-trivial linear combination of these vectors which is equal to zero vector 0. Otherwise the vectors are called independent. A most important characteristic of a vector space V is the maximal number of linearly independent vectors in it. This number is called the dimension of V, is denoted by dim V and takes values {0, 1,,..., }. 1 Two finite dimensional vector spaces over the same field are isomorphic iff their dimensions are equal. Let V 1 and V be the vector space over a field K. Definition 3.3. A map A : V 1 V, v Av is called linear if it has the property The linear maps are also called linear operators. A(λv + µw) = λav + µaw. (3.1) 3.. Modules over a ring. A generalization of the notion of a vector space over a field K is the notion of module over a ring R. More precisely, for noncommutative ring we have to distinguish the left modules and right modules. Often we restrict ourselves by left modules and call them simply modules. Definition 3.4. A left module over the ring R is an abelian group (M, +) endowed by the left action of a ring R, i.e. there is a map satisfying two distributive laws R M M : (a, m) am a(m 1 + m ) = am 1 + am ; (a + b)m = am + bm; (ab)m = a(bm) and the action condition (ab)m = a(bm). In other words, an element a of the ring R acts on the module M by the transformation L(a) : m am. The action condition means that L(ab) = L(a) L(b). 1 Actually, there are different kinds of infinity, but in our course we do not go deep in the question.

8 8 A.A.KIRILLOV For example, any abelian group (M, +) has a canonical structure of a Z- module. Namely, the action of n Z on the element m M gives n times {}}{ m + m + + m if n > 0, nm = 0 if n = 0, n m if n < Algebras. The last object of our Zoo is a notion of algebra. We call a set A an algebra over the field K if it is simultaneously a ring (A, +, ) and a vector space over K; these two structures are related by the condition that the ring multiplication is a bilinear operation: (λa + µb) c = λac + µbc; a (λb + µc) = λab + µac. Algebras, as rings, can be associative, commutative or division algebras but in general do not possess any of these properties. The most important example of an associative algebra over a field K is a matrix algebra Mat n (K) with the natural structures of a ring and a vector space over K. For algebras, as for rings, the notion of a module makes sense. Namely, a vector space M is a left module over an algebra A (both over the same field K), if a bilinear map A M M : (a, m) am is defined and satisfies the usual axioms of a module. Example. Consider the n-dimensional vector space V over K consisting of column vectors v of size n with entries v i, 1 i n, from K. It is a module over Mat n (K) with respect to the action n (Av) i = a ij v j. j=1 So, a matrix A defines a linear operator L(A) in the space V. Actually, the multiplication of matrices in (.1) above is defined specially to have the property L(A) L(B) = L(AB), i.e. to be compatible with the composition of operators. For algebras, as for rings, the notions of a left, right and two-sided ideal make sense. For any two-sided ideal I A a quotient, or factor algebra A/I is defined. 1 Department of Mathematics, University of Pennsylvania, Phila, PA ,USA. address: kirillov@math.upenn.edu